Purines
and Pyrimidines are the nitrogen-containing "bases" found
in the nucleotides that make up DNA and RNA. As shown in Figure
12-5 from the Dragonfly Book (at right), purines have two rings
in their structure, while pyrimidines have just one.

Unfortunately,
while this Figure is correct in all editions of the book, early printings
of the Dragonfly Book had a typographical error on Page 291 that reversed
the identifications of these two types of bases. All printings of
the book since June of 2002 should contain the corrected text.